When a mechanical shock or vibration is imparted to the outside of an operating electric lamp employing a coiled incandescent filament, the force of the impact may cause the lamp filament to sway or oscillate. If an operating filament is perturbed sufficiently so as to come into contact with one of the electrical lead-in wires between which the filament is connected, the filament will be electrically shorted which likely will result in premature failure of the filament and lamp.
In a lamp having a coiled filament positioned near the envelope wall, as is typical in many tungsten halogen lamps, premature lamp failure may occur as a result of the operating filament coming into contact with the glass wall. The thermal shock imparted by the hot filament may crack the glass envelope and cause lamp failure, possibly a violent failure.
The effect of gravity on a coiled filament, i.e., filament sag, may also give rise to premature lamp failure via shorting the filament or thermal shocking the glass envelope. When mounted horizontally, the filament has a catenary shape between filament supports. If jostled sufficiently, the middle section of an operating filament may come in contact with a lead-in wire or the envelope wall. When mounted vertically, numerous turns of filamentary coil may bunch near the lower end of the filament. If jostled sufficiently, a number of coil turns of an operating filament may come into electrical contact with each other resulting in a shorting of the filament.
The problem of filament sway or sag increases in severity as the length of the coiled filament increases and the diameter of the filamentary wire decreases. In order to achieve commercially feasible performance, such as lumen output and filament life, the design length and wire diameter of the filament are rather narrowly constrained. In filaments having design operating voltages of two hundred volts or higher, e.g., lamps for use in many European countries, commercially feasible filaments necessitate longer filaments with equivalent or thinner diameters than required in their lower voltage counterparts. Accordingly, additional or intermediate filament supports are commonly employed where filament sway or sag is a concern.
Use of an intermediate filament support is not without certain disadvantages, particularly with multiple coil filaments. Typically, an intermediate support is a hook or loop of wire having significantly more mass than the filamentary wire; the support hooks or grasps the filament. Mounting a filament on the heavier support may distend the filament turns near the mount. The relatively thick support wire may electrically short out two or three outermost turns of the filament, which may shorten the life of the filament. The more massive support may cool the filament appreciably in the vicinity of the mount which in turn may affect beneficial chemical reactions, such as the tungsten halogen cycle, between the operating filament and its environment.
In order to avoid these disadvantages, a loop of wire surrounding but not touching the filament has been employed as an intermediate support. While this type of support may restrain filament movement outside of the loop's perimeter, it does not deter movement along the filament's axis.
It would be a substantial advancement of the electric lamp art if an intermediate filament support were provided which substantially reduces one or more of the disadvantages associated with intermediate filament supports of the prior art, particularly if the new support may be employed without increasing the cost of lamp manufacture.
The following prior art patents are believed to be pertinent to the invention. In U.S. Pat. No. 4,743,803, issued May 10, 1988, to Lanese et al., particularly in FIGS. 10 and 11 thereof, there is disclosed an intermediate filament support which encircles the filament. The support may be mounted by means of a glass beam formed around one of the lead-in wires or by imbedding one end of the support in the press seal of the lamp's envelope. This patent contains a discussion of the filament shorting problem caused by mechanical shock or vibration.
The Martin patent, U.S. Pat. No. 3,930,177, issued Dec. 30, 1975, shows an hook-type intermediate filament support which is mounted in a glass bridge within the lamp's envelope.
An example of an intermediate filament support for a tubular incandescent lamp is shown in Morris et al., U.S. Pat. No. 4,359,665, issued Nov. 16, 1982. Also see U.S. Pat. Nos. 3,194,999, issued July 13, 1965, to Heinlein; 3,335,312, issued Aug. 8, 1967, to Cardwell, Jr.; 3,634,722, issued Jan. 11, 1972, to Palmer et al.; 3,678,319, issued July 18, 1972, to Notelteirs et al.; 3,736,455, issued May 29, 1973, to Notelteirs et al.; and 3,891,885, issued June 24, 1975, issued to Wurster; for further examples of intermediate supports in tubular lamps.
Two patents to Graves et al., U.S. Pat. Nos. 4,316,116, issued Feb. 16, 1982, and 4,499,401, issued Feb. 12, 1985, are noted for their discussions of single, double, and triple coiled filaments.